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gruntstripe

400kw motor winding resistance

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Hi, I had an ABB ACS800 inverter problem yesterday: the drive tripped at start up with fault code 2340 (short circuit). The manual suggested testing both the motor windings and the supply cable. Using my Fluke the windings only measured about 0.1R which is only the meter lead resistance I presume. I couldn't determine if there was definately a short or not - the armature winding tails were large cross section so they probably would measure next to nothing. Basically I'm asking what you would expect such a large motor to read and if it is only a very small reading how could one determine if the windings were short or not? As it happens, and after consulting ABB, the fault turned out to be on the drive electronics so we frigged the parameters to keep it going. Thanks in advance.

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I don't think you can test the motor's windings with a regular DMM, for the reason you just mentioned. You need a meter that is accurate in milliohms or even microohms. Special equipment is made for this. (Note that I don't have any first-hand experience using this kind of stuff, though)

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I don't think their asking for the motor winding resistance. Check for shorts to ground or motor casing.

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Yep. And also measure between windings. Remember to remove the supply leads (doh). edit: And also remove the bars that connect the windings in Y or D (double doh !). Edited by JesperMP

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In answer to the original post, the standard test for a motor is to use a megger. "Megger" is the brand name but almost every major brand name in instruments sells an insulation resistance tester of some sort. It reliably detects about 85-90% of the motor failures. There are no false positives. Essentially what you do with a megger is to use it like a high voltage ohm-meter, checking resistance to ground with a voltage that is darned close to your actual line voltages (in case of a dielectric-type fault where a standard meter won't arc over the air gap). The "correct" practice is to directly test at the motor in isolation. However, you can test the entire motor branch circuit if you isolate at the overload block first. Then if you get a good reading, it's either a "10% case" or something else. This test takes just a few minutes on a motor at <600 volts and mechanical failures are so common that it's usually a good first step. It's very fast compared to how much work you may have with taping and splicing at the motor itself. If I get a bad reading, I go ahead and open up the motor leads directly at the motor termination enclosure (peckerhead). This isolates the motor from the circuit going to it. If I get a second bad reading, I know it's the motor. If not, then I've already isolated the problem to something in the motor branch circuit leads. The stated purpose of an insulation resistance test is obviously to test for degradation of the insulation. However, if the bearings go, the motor gets wet, or you get a short across the rotor or the field coils mechanically fail...the things that typically happen to a motor, the air gap resistance drops down to just about nothing and you will detect it. The general rule is that a motor should have at least 1 mega-ohm of resistance, or 1 mega-ohm per 1 kV, whichever is higher...and higher is always better. The false "negatives" that I've found on AC squirrel cage motors usually involve either a bad inertia switch if it has one, or else on occasion I'll find one with a strange magnetic center. Either one will only appear while the motor is in operation but not when it is dead still. Question #1: Why? It really doesn't matter if you can isolate which coil has the problem. The issue is whether there is a fault to ground somewhere in the motor, or not. In practice, I just take all the leads and short them together when doing a megger test in case I have a faulted coil or faulted leads between coils. Question #2: Is a "megohm" reading with an ordinary multimeter (even one specially modified for the high resistance range) even close to accurate? Reason being that the "online" or "offline" insulation resistance monitors don't actually measure using a voltage source close to the actual usage. Most use a DC voltage somewhere around a few volts. The theory is that you may not have a high enough voltage to get insulator arc-over on a marginal circuit so that the various low voltage detectors will miss what the high voltage ones find. But I can't find any documentation proving it out either way.

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I've found that an old fashioned analog Simpson or heaven forbid a megger works best when reading motor windings. These new fangled fluke DVM seem to have some problem when Resistance and Inducatance are present at the same time.

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How do you check for isolation problems between the windings without isolating the windings from each other ?

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Hi, Thanks for your replies. I'm sorry if I didn't make my original post clear enough. I am familiar with motor testing routines (I'm a maintenance Tech'). The nub of the issue is how to assess if a particular winding on a very large ac motor is short - but not to ground. Most motors I usually deal with are pretty small, maybe just a few kw and if a resistance reading is taken across a winding a descernable result is obtained, say like 10R. So one would have three 10R readings (U1, V1, W1) on a good motor. If a winding is internally shorted the reading would be 0R. But on a very large motor of say 400kw rating the cross sectional area of the winding wires is large and the resisance is extremely small by default, I presume. Thus how can one determine if the winding is internally shorted? I have never had to test a motor this large before. I think gravitar had a good answer.

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Gruntstripe, I think Lancie1 has a partial answer to your question over on the other forum, post #43 http://www.plctalk.net/qanda/showthread.php?t=434. Im not suggesting the procedure for identifing leads but using the transformer effect he talks about and measuring the voltages. This should give you a good idea if some of your windings have lost some turns due to a short inside them and not to ground. edit: Today 504Bloke added to this 4 year old thread (mentioned above) this link http://www.martindaleco.com/HTML/Electrica...llTestPro31.htm This looks to be exactly what you want. Edited by milldrone

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